The present invention pertains to automatic chroma circuits in color television receivers. More particularly, it relates to a sample-and-hold circuit with quick recovery time especially adapted for use in automatic chroma circuits.
Most color television receivers include automatic control systems for achieving stability of operation and fidelity of reproduction in the presence of variations in the received television signal. For example, the well-known automatic gain control (AGC) responds to the amplitude of the video carrier to develop a control signal for the preceding turner and intermediate-frequency amplifier to maintain a constant carrier amplitude at the video detector. Many receivers also have an automatic frequency control (AFC) for correcting any deviation in the video IF carrier from its design-center value. It is also very common to include another automatic frequency control for maintaining synchronization of the horizontal scanning circuits with the horizontal synchronizing pulses in the received television signal.
Color receivers also include means for synchronizing the receiver regenerated color reference subcarrier with the transmitted 3.58 megahertz color burst signal for faithful reproduction of the hue (tinting) in the video display. Another conventional form of automatic control in color receivers is that called automatic chroma or color control (ACC). Here, the gain of the chroma amplifier is varied to maintain the amplitude of the burst signal constant and a viewer adjustable control is provided for adjusting the chroma gain to achieve an average desired color level in the reproduced picture to his own preference.
In spite of the many control circuits mentioned, inconsistency in chroma content of the reproduced image persists, because the chroma content or color level of the picture is not consistently related to the burst amplitude. The difficulty is particularly evident when the receiver is of the recently introduced, and well accepted, type which includes a switch for selecting preset factory adjustments of different parameters in preference to the manual controls. That is, the receivers include a number of normally viewer-inaccessible controls which are preset to obtain a technically optimum display. The viewer may, of course, use the manual controls by operating the switch. Because of the inconsistent relationship of the chroma content and color burst, however, even the factory set color level control adjustment is unable to give satisfactory results for the wide variety of color levels encountered.
In the above-mentioned abandoned application, a television receiver is provided with a preset/manual switch and an automatic color level limiter which comprises a sample-and-hold circuit arrangement in a closed loop including the chroma amplifier. The circuit arrangement includes a peak detector for detecting color signals in excess of a predetermined threshold and means producing compensatory gain adjustments on the chroma amplifier over a relatively long time interval. Effectively, the ACL system produces correction information based upon above-threshold peaks in color level, the information being updated by subsequent peaks. The ACL loop contains a transistor controlled preset color level potentiometer. The transistor is normally operated near saturation and, because of the closed loop, could not be driven to cutoff.
With the receiver operated with the manual/preset switch in the manual position, however, the ACL closed loop is open and the transistor can be driven beyond cutoff. Subsequently switching to preset operation often resulted in a long recovery time before the ACL loop regained control. The preferred embodiment disclosed a Miller effect capacitor coupled across the collector and base of the transistor for overcoming this problem, which is the subject of this application. Because of this interrelationship abandoned application Ser. No. 347,932 filed Apr. 4, 1973 in the names of Charles H. Heuer and Howard F. Jirka is hereby incorporated by reference in the present application.